The present invention relates to a device and to a sensitive medium for measuring an absorbed dose in an ionizing radiation field. More particularly, the invention relates to a device for determining the magnitude of the absorbed dose at a specific point in a material where the interaction of the radiation with the material resembles the interaction of the radiation with living tissue. The present invention also relates to the use of a sensitive medium, such as a suitable fluid mixture, in such a device.
By absorbed dose is meant the energy that is delivered by the ionizing radiation per unit of mass of the radiated material. The unit of an absorbed dose is the gray, abbreviated herein as Gy (J/kg).
With view to patient safety, for instance in connection with medical radiation treatment and radiological diagnosis, it is of paramount importance to be able to anticipate the biological effects on living human tissue that is subjected to radiation in an ionizing radiation field. In this regard, it is desirable to be able to predict the dose absorbed and its distribution in human tissue before irradiating the tissue. It is of interest to measure the delivery of the ionized radiation at different proximate points, so as to map the ionizing radiation field. Patient safety can be enhanced by accurate and repeated mapping of the radiation field in treatment apparatus.
Examples of ionizing photon radiation fields and electron radiation fields that are relevant in medical treatment and radiological diagnosis are photon and electron radiation fields in the energy range of from 30 keV to 50 MeV. A dose rate interval of interest in continual radiation is from 0.1 mGy/min. to 10 Gy/min. and in the case of pulsed radiation with microsecond pulses, an absorbed dose per pulse of up to 5 mGy/pulse and a pulse repetition frequency of up to 1,000 pulses per minute. By dose rate is meant the rate at which the dose grows, i.e. the energy delivered per unit of mass and per unit of time.
The energy deposited/dose absorbed by a patient in computerized tomography, for instance, can be predicted by providing an artificial body that includes a tissue-like material (normally water) with detectors and subjecting said body to the ionizing radiation field. With the use of good detectors, the absorbed dose and its distribution can be interpreted from signals from the detectors, combined with the knowledge of the placements of said detectors. When a patient is then subjected to the same radiation field under generally the same conditions (inter alia energy and time), the dose delivered to the patient can be predicted.
The most common type of detector at present within the described field of application is an ionization chamber that uses a gas as the sensitive medium. The ionization chamber operates on the principle of irradiating a chamber-enclosed sensitive medium, wherein ions are released in proportion to the energy delivered as a result of the interaction of the radiation with the gas. These ions are captured via an electric field generated between two electrodes. The captured charge can then be measured and used to determine the size of the dose absorbed in the sensitive medium.
One drawback with these detectors is their size, more specifically the size or volume required to enable the gas to be used as a sensitive medium with sufficient accuracy. Furthermore, since the detector responds to the total amount of energy deposited in the sensitive medium, local energy spikes in the radiation field will be concealed, therewith rendering the detector unusable or unreliable when such variations in energy density are found in the ionizing radiation field. Variations that occur in radiation fields derive from many present-day radiation sources. For instance, there can occur points in the patient's tissue where the dose actually absorbed differs widely from the predicted dose.
Semi-conductors diodes are examples of other conventional detectors. These detectors operate on the principle whereby the ionizing radiation generates free charges in the p-n-junction of the diodes. Similar to the ionization chamber technique, this charge can then be used to determine the dose absorbed.
A common feature of these types of detector is that there is measured an electric current or charge that is proportional to the dose rate and the absorbed dose respectively. The measuring result can thus be registered directly during or immediately after irradiating the detector.
Another class of detectors for measuring absorbed dose are those which enable registration of a change in the detector material caused by radiation, subsequent to irradiating the detector. One example of the latest type of detector is the so-called Fricke dosimeter, which uses spectrophotometric evaluation of the irradiated liquid. One drawback with detectors of this type is that they cannot be made sufficiently small with the sensitivity required in the application that we have described in this document.
Another type of detector is the so-called thermoluminescence dosimeter which utilizes the phenomenon that irradiation of certain materials causes a specific amount of electrons excited by the radiation to remain in the material in their excited state. When the irradiated material is subsequently heated, the electrons are de-excited and the quantity of light thus generated is proportional to the dose absorbed by the material under certain conditions.
These latter detectors do not permit a dose response to be read directly, and the proportionality between emitted signal and absorbed dose in water varies in the best of cases by about 40% in the given energy range. The semiconductor detector can be produced in small units (only some millmeters) and has a variation of about 500% in its response in the aforesaid energy range of interest. Such variations are unsatisfactory.
The need of reliable and sensitive detectors in the photon energy range of 30-200 keV that also have a limited extension in space is particularly obvious when the purpose of such detectors is to determine absorbed doses and dose distribution around small radioactive radiation sources intended for implantation in cancer tumours for radio therapy (interstitial radio therapy), a tumour treatment form that has been revived in recent years.
Another important field of application is one of determining the size of the absorbed dose and its distribution in patients undergoing computerized tomography. In Sweden, as in most other countries, it is a statutory requirement that doses are measured at regular time intervals, to guarantee safety of the patients.
The object of the present invention is to provide a device for measuring doses absorbed in a material that is subjected to ionizing radiation, particularly in a tissueimitating material.
A further object is to provide a device that
a) has a radiation sensitive volume that is small in relation to the variation of the absorbed dose in space. In practice, this means that the sensitive volume of the detector will preferably not have in any direction an extension that exceeds a few millmeters. Certain applications require the sensitive volume to have an extension of less than some tenths of a millimeter in at least one direction; PA1 b) by its presence in an irradiated material only negligibly disturbs or perturbates the radiation field that would otherwise be available; PA1 c) ensures that the proportionality between measuring signal and the absorbed dose in the material where measuring shall take place is not noticeably changed with variations in the energy spectrum of the radiation (radiation quality variations); PA1 d) does not permit the proportionality between measuring signal and absorbed dose to vary markedly with the dose rate or the size of the absorbed dose; and that PA1 e) it ensures that the accuracy in determining the absorbed dose, particularly in the case of measurements on which radio therapy is to be based, is greater than some percent.
If condition c) is not fulfilled, accurate knowledge of the radiation energy spectrum is required in order to measure the dose reliably. Energy spectra often vary in the voids in an irradiated object and are equally as difficult to determine as the absorbed dose.
These and other objects of the invention are achieved with an inventive device defined in the characterizing clauses of respective Claims.